Field of the Invention
The present invention relates to a multiplex optical communication system for transmitting multiplex optical signals under a signal transmission technology of Wavelength Division Multiplexing (WDM) or Optical Time Division Multiplexing (OTDM).
In particular, the present invention relates to optical amplifiers for amplifying power of the multiplex optical signals transmitted through the system.
FIG. 1A is a block diagram for illustrating the multiplex optical communication system of the related art, operating under the WDM or OTDM technology. The multiplex optical communication system is composed of optical signal terminating equipment (TERM EQUIP) (1 and 1xe2x80x2) placed at both terminals of the OPT-TRANS LINE 3, for transmitting and receiving the multiplex optical signals, an optical transmission line (OPT-TRANS LINE) (3) made of an optical fiber depicted by a thick line, for transmitting the multiplex optical signal, and optical amplifier repeater equipment (OAMP REP EQUIP) (2) placed along the OPT-TRANS LINE 3, for amplifying and repeating the multiplex optical signals transmitting between the TERM EQUIP 1 and 1xe2x80x2.
The TERM EQUIP 1 includes a transmitting unit (TX-UNIT) (1001) for transmitting a multiplex optical signal to the TERM EQUIP 1xe2x80x2 and a receiving unit (RX-UNIT) (1002) for receiving a multiplex optical signal transmitted from TERM EQUIP 1xe2x80x2. The same as TERM EQUIP 1, the TERM EQUIP 1xe2x80x2 includes TX-UNIT 1001xe2x80x2 and RX-UNIT 1002xe2x80x2. The TERM EQUIP 1 and TERM EQUIP 1xe2x80x2 have the same constitution and function, so that the TERM EQUIP 1 will be representatively described hereinafter. The OAMP REP EQUIP 2 includes two repeater optical amplifiers (REP OPT-AMPs) (8 and 8xe2x80x2). The REP OPT-AMP 8 is to amplify the multiplex optical signal transmitted from the TX-UNIT 1001 in the TERM EQUIP 1, for repeating the multiplex optical signal to RX-UNIT 1002xe2x80x2 in the TERM EQUIP 1xe2x80x2, and the REP OPT-AMP 8xe2x80x2 is to amplify the multiplex optical signal transmitted from the TX-UNIT 1001xe2x80x2 in the TERM EQUIP 1xe2x80x2, for repeating the multiplex optical signal to RX-UNIT 1002 in TERM EQUIP 1. When a distance between TERM EQUIP 1 and 1xe2x80x2 is long, a plurality of the OAMP REP EQUIP 2 are placed. However, one OAMP REP EQUIP 2 is representatively depicted in FIG. 1A.
FIG. 1B shows a block diagram of the TX-UNIT 1001 of the related art. The TX-UNIT 1001 consists of electro-optical signal converter (ELEC-OPT CONV) (4) connected with electrical signal channel lines (ELEC-SIG CHANNEL LINEs) (9) through which a plurality of electrical signals formed to channels are sent to the ELEC-OPT CONV 4, an optical signal combiner (OPT-SIG COMB) (5) connected with the ELEC-OPT CONV 4 through optical fibers depicted by thick lines, and a transmitting unit optical amplifier (TX-UNIT OPT-AMP) (6) connected with the OPT-SIG COMB 5 through an optical fiber depicted by a thick line. The ELEC-OPT CONV 4 is for converting the electrical signals to optical signals at every channel. The ELEC-OPT CONV 4 consists of converters 4-1, 4-2, - - - and 4-n in correspondence with the ELEC-SIG CHANNEL LINEs 9. When the electrical signals are fed to the ELEC-OPT CONV 4 through the ELEC-SIG CHANNEL LINEs 9, the converters 4-1, 4-2, - - - and 4-n convert the electrical signals to optical signals and send the optical signals to the OPT-SIG COMB 5, respectively. The OPT-SIG COMB 5 is for combining the optical signals sent from the ELEC-OPT CONV 4, adopting the WDM technology or the OTDM technology, so as to produce a multiplex optical signal. The TX-UNIT OPT-AMP 6 is for amplifying the power of the multiplex optical signal sent from the OPT-SIG COMB 5. The amplified multiplex optical signal is sent out from the TX-UNIT 1001 to the REP OPT-AMP 8 in OAMP REP EQUIP 2 through the OPT-TRANS LINE 3.
FIG. 1C shows a block diagram of the RX-UNIT 1002. The RX-UNIT 1002 consists of an optical signal branching unit (OPT-SIG BRANCH) (5xe2x80x2) and optical-electro signal converters (OPT-ELEC CONVs) (4xe2x80x2). The OPT-SIG BRANCH 5xe2x80x2 is connected with the OPT-TRANS LINE 3 depicted by a thick line, for optically demultiplexing the received multiplex optical signal to a plurality of received optical signals which are called xe2x80x9creceived demultiplexed optical signalsxe2x80x9d hereinafter. The received demultiplexed optical signals produced at the OPT-SIG BRANCH 5xe2x80x2are sent to the OPT-ELEC CONV 4xe2x80x2 through optical fibers depicted by thick lines. The OPT-ELEC CONV 4xe2x80x2 consists of converters 4xe2x80x2-1, 4xe2x80x2-2, - - - , 4xe2x80x2-n at which the received demultiplexed optical signals are converted to received electrical signals and sent out from RX-UNIT 1002 to the ELEC-SIG CHANNEL LINEs 9, respectively.
In FIG. 1A, the OAMP REP EQUIP 2 includes two optical amplifiers (8 and 8xe2x80x2) which will be called REP OPT-AMPs 8 and 8? hereinafter. The REP OPT-AMP 8 and 8xe2x80x2 are for amplifying the power of multiplex optical signals received from TERM EQUIP 1 and 1xe2x80x2,respectively. By virtue of the REP OPT-AMPs 8 and 8xe2x80x2, power loss, caused by the OPT-TRANS LINE 3, of the multiplex optical signals transmitting between TERM EQUIP 1 and 1xe2x80x2 are recovered. Therefore, when a length of the OPT-TRANS LINE 3 between TERM EQUIP 1 and 1? is long, a plurality of the OAMP REP EQUIP 2 are placed along the OPT-TRANS LINE 3, and the number of the OAMP REP EQUIP 2 is determined by considering both the power loss due to the OPT-TRANS, LINE 3 and the power amplification factors of REP OPT-AMPs 8 and 8xe2x80x2 in GAMP REP EQUIP 2, so that the multiplex optical signals can be transmitted between the TERM EQUIP 1 and 1xe2x80x2 in high fidelity and a high signal to noise ratio (SNR).
Generally, there are two kinds of optical amplifiers, a semiconductor amplifier and an optical fiber amplifier. The both kinds of optical amplifiers can be applied to the TX-UNIT OPT-AMP 6 in FIG. 1B and the REP OPT-AMPs 8 and 8xe2x80x2 in FIG. 1A.
For example, in case the TX-UNIT OPT-AMP 6 is the semiconductor amplifier, the multiplex optical signal fed to the TX-UNIT OPT-AMP 6 is amplified by a semiconductor device operating under DC supply current, and in case the TX-UNIT OPT-AMP 6 is the optical fiber amplifier, the multiplex optical signal fed to the TX-UNIT OPT-AMP 6 is amplified in an optically amplifying technology using an induced emission.
Recently, the optical fiber amplifier is used to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 8 mostly. Because, the optical fiber amplifier has features such as a low Noise Figure, a little non-linearity in amplification, a low connection loss with the OPT-TRANS LINE 3, high capability of a power amplification and a high stability against a temperature change.
The optical fiber amplifier is composed of a rare earth metal-doped optical fiber such as Erbium (Er)-doped optical fiber and a pump light source such as a semiconductor laser.
In the multiplex optical communication system of the related art, the output power of the multiplex optical signal from the TX-UNIT OPT AMP 6 or the REP OPT-AMP 8 is controlled so as to be always constant in level under constant output level control performed in the TX-UNIT OPT AMP 6 and the REP OPT-AMP 8 respectively. In case of the REP OPT-AMP 8, by virtue of the constant output level control, the OAMP REP EQUIP 2 can be placed independently on a length of the OPT-TRANS LINE 3 connected with the OAMP REP EQUIP 2. In other words, the power level of each section between REP OPT-AMPs or between TERM EQUIP and REP OPT-AMPs is independent. The change of power level and different of OPT-TRANS LINE 3 loss at one section don""t affect the power level at next section.
If a multiplex optical signal includes xe2x80x9cnxe2x80x9d channels and the TX-UNIT OPT-AMP 6 is required to produce at least output power xe2x80x9cPoxe2x80x9d per a channel for obtaining an advisable SNR, the TX-UNIT OPT AMP 6 must be designed so as to produce output power of xe2x80x9cPoxc3x97nxe2x80x9d. In other words, the TX-UNIT OPT AMP 6 initially produces the optical output under the constant output level control so that the output power of the TX-UNIT OPT AMP 6 corresponds to the number of the channels of a multiplex optical signal to be initially amplified by the TX-UNIT OPT AMP 6.
From a viewpoint of the operational flexibility of the multiplex optical communication system, it is desirable that the channels of the multiplex optical signal can be changed easily in response to trouble about the transmission of the multiplex optical signal and up grade of trafic capacity. For example, at first some channels which meet demand are used. When more traffic capacity are needed, other channels will become used. Usually, the multiplex optical communication system provides at least one spare channel in place of a fallen channel. For example, when a module of a channel has trouble, another module of the spare channel is used instead of the troubled module. Such previous provision of the spare channel is effective for increasing the operational reliability of the multiplex optical communication system. However, when the spare channel is used, there has been a problem of the output power in the multiplex optical communication system of the related art.
In the multiplex optical communication system of the related art, the constant output level control is performed to optical amplifier so as to keep the total output of the multiplex optical signal constant. As a result, when the number of the channels decreases by removing a CONV which will be called xe2x80x9cremoved CONVxe2x80x9d hereinafter, single output of each channel increases. On the contrary, when the number of the channels increases by adding a CONV which will be called xe2x80x9cadded CONVxe2x80x9d hereinafter, the single output of each channel decreases.
When output power of a channel of the multiplex optical signal changes thus, a problem due to a non-linear effect occurs on the optical fiber of the OPT-TRANS LINE 3. That is, when the power of a channel exceeds a specific level, a waveform of each channel is distorted by the non-linear effect on the optical fiber. The non-linear effect is generally called a self phase modulation effect. Meanwhile, in contradiction to the above, the power of the optical signal is required to be larger than a specific level for maintaining a required SNR at the receiving unit such as the RX-UNIT 1002 or 1002xe2x80x2.
In the REP OPT-AMP 8, minimum input power and maximum output power are required for performing the reception and the transmission of the multiplex optical signal safely. When there are a plurality of the REP OPT-AMPs 8 in the multiplex optical communication system, these minimum input power and maximum output power are determined by the reception and amplification ability of each REP OPT-AMP 8 and the number of the REP OPT-AMPs 8. In each REP OPT-AMP 8, a level difference between the minimum input power and the maximum output power is called a transmission and reception level difference. The REP OPT-AMP 8 is designed so that the transmission and reception level difference is larger than a signal loss caused by the OPT-TRANS LINE 3 lying between the OAMP REP EQUIP 2. Furthermore, in the design of the REP OPT-AMP 8, a margin of output power of each channel is afforded to insure its level difference caused by increase or decrease of the number of the channels in the multiplex optical signal. Because of allowing the margin thus, a share of the transmission and reception level difference to the optical transmission loss is decreased. In other words, a distance between the OAMP REP EQUIP 2 is shortened. This results in increasing the number of the OAMP REP EQUIP 2 uneconomically.
Therefore, an object of the present invention is to make the multiplex optical communication system have a large operational flexibility against the variation of the total number of the channels in the multiplex optical signal transmitted through the system.
Another object of the present invention is to increase the operational fidelity of the multiplex optical communication system.
Still another object of the present invention is to improve contradiction occurring in the system that increasing output power is required to maintain the required high SNR, however, the output power cannot be increase so high because the self phase modulation effect occurs in the system.
Further another object of the present invention is to decrease costs for constructing and maintaining the repeater equipment by decreasing the number of the repeater equipment.
The above objects are achieved by controlling the terminal equipment and the repeater equipment of the system so that output variation occurring at the terminal equipment is made equal to the output variation occurring at the repeater equipment. Wherein, the converter is provided in the terminal equipment for converting an electrical signal to an optical signal. The multiplex optical signal is formed by combining the converted optical signals produced at the converters in corresponding to a plurality of the electrical signals fed to the terminal equipment. The output variation is caused by changing the number of operating converters due to adding or removing a converter to or from the operating converters.
In order to make the optical output variations equal to each other, either of two ways is operated to the optical amplifier of the repeater equipment while the added converter is increasing output or the removed converter is decreasing output. One way is xe2x80x9cto change the output power of the optical amplifier under the same time-constant as the output change of the converterxe2x80x9d which is called xe2x80x9ctime constant controlxe2x80x9d. Another way is xe2x80x9cconstant gain controlxe2x80x9d which is control for keeping gain of the optical amplifier constant.
First of all, xe2x80x9ctime constant controlxe2x80x9d will be explained.
The time constant at changing the target of output level of the optical amplifier is set as same as the time constant of the added converter or the removed converter, then the output power of each converter is kept constant even when converter is added or removed newly. Before adding or removing a converter, the terminal and the repeater equipment produce output under xe2x80x9cconstant output level controlxe2x80x9d which is for keeping output of the multiplex optical signal produced from the terminal and the repeater equipment, constant. When a converter is added or removed, the optical amplifier in the repeater equipment changes the target of the prescribed value under the same time constant as the output change of the converter. In order to perform changing the target of output level and announce the prescribed value, a monitor controller is provided to the terminal equipment for generating an optical output control signal to be sent to the optical amplifier in the repeater equipment.
When a converter is added or removed, the monitor controller monitors the output of the converters and prohibits that an added or removed converter starts to increase or decrease the output and sends the optical output control signal to the optical amplifier in the repeater equipment. After that, the monitor controller permits that the added or the removed converter starts to raise or decrease the output power. The optical amplifier receives the optical output control signal and starts to change the target of output level to the prescribed value which is information on the optical output control signal announced from the monitor controller.
The prescribed value is proportional to the number of the converters at the terminal equipment.
Next xe2x80x9cconstant gain controlxe2x80x9d will be explained. Before adding or removing the converter, the optical amplifier of the repeater equipment produces output under xe2x80x9cconstant output level controlxe2x80x9d which is for keeping output of the multiplex optical signal produced from the terminal and the repeater equipment, constant.
When a converter is added or removed, the monitor controller which monitors the number and the output power of converters, prohibits that the added or the removed converter starts increasing or decreasing output, and sends the optical output control signal to the optical amplifier. After that, the monitor controller permits the added or the removed converter starts increasing or decreasing the output power. Upon receiving the optical output control signal, the optical amplifier changes xe2x80x9cconstant output level controlxe2x80x9d to xe2x80x9cconstant gain controlxe2x80x9d.
After the added or the removed converter finishes increasing or decreasing the output power, the control of the optical amplifier returns to the xe2x80x9cconstant output level controlxe2x80x9d from xe2x80x9cconstant gain controlxe2x80x9d. However, at this time, a constant output level is different from the previous one, because it is a prescribed value determined by the number of the converters. This is announced to the optical amplifier by the optical output control signal.
This switching from xe2x80x9cconstant gain controlxe2x80x9d to xe2x80x9cconstant output level controlxe2x80x9d is performed by the optical output control signal or automatically, by no signal, after time, which is enough for the added or the removed converter to finish increasing or decreasing the output power, passed.
When the optical amplifier produces output under the time constant control, a delay occurs between the output from the added or removed converter and the output from the optical amplifier. The delay is shortened by using step-by-step time constant control which is performed by providing half way objective values in the monitor controller so that the added or the removed converter produces output step by step under the previously determined rising or falling time constant. Every time the added or removed converter starts and stops increasing or decreasing output, the optical output control signal including the start information is sent from the monitor controller to the optical amplifier. As a result, the optical amplifier repeats starting and stopping the increase or decrease of the output corresponding to the objective values. By virtue of the set-by-step control, the error due to the delay is reduced on an average.
The optical output control signal is transmitted between the terminal equipment and the repeater equipment through an optical transmission line connecting them.
The improvement described above is performed to the multiplex optical communication system operating under Wavelength Division Multiplexing (WDM) or Time Division Multiplexing (OTDM).